pumping speed — Pumping speed refers to the volume flow rate of a pump at its inlet, often measured in litres per second, cubic feet per minute, or cubic metre per hour. Because of compression, the volume flow rate at the outlet will always be much lower than at the inlet. Momentum transfer and entrapment pumps are more effective on some gases than others, so the pumping speed can be simultaneously different for each of the gases being pumped, and the average pumping speed will vary depending on the chemical composition of the gases remaining in the chamber.

References: Wikipedia, the free encyclopedia – http://en.wikipedia.org/w/index.php?title=Vacuum_pump\